Pressure-sensitive adhesive comprising natural rubber and a styrene-methyl methacrylate-ethyl acrylate terpolymer



United States Patent Oflice 3,3033% Fatented Apr. 30, 1968 3,380,938PRESSURE-SENSITIVE ADHESIVE COMPRISING NATURAL RUBBER AND ASTYRENE-METH- YL METHACRYLATE-ETHYL ACRYLATE TER- POLYMER James Jack,Mistley, Essex, and James Anthony Horrocks, Ipswich, Sufiolk, England,assignors to B.X. Plastics Limited, London, England, a British companyNo Drawing. Continuation-impart of application Ser. No. 282,164, May 21,1963. This application June 15, 1966, Ser. No. 557,609

2 Claims. (Cl. 260-4) ABSTRACT OF THE DISCLOSURE A pressure-sensitiveadhesive for a polyolefin film comprising a natural rubber latex and anaddition polymer latex, the weight of the solid addition polymer beingin the range 20-80% of the solid material present and the additionpolymer being capable of keying to a polyolefin film, being non-tackyand incompatible with natural rubber.

This application is a continuation-in-part of U.S. application Ser. No.282,164, filed May 21, 1963. It relates to adhesive compositions. Itparticularly relates to pressure-sensitive adhesives based on naturalrubber latices. By a pressure-sensitive adhesive is meant throughout thespecification an adhesive which when applied to a surface adheresthereto to give a coating which does not adhere strongly to uncoatedsurfaces but does adhere to another similarly coated surface when thetwo surfaces are brought together under pressure. Suchpressure-sensitive adhesives are well-known; for example the naturalrubber latex based adhesives.

The present invention is a pressure-sensitive adhesive for a polyolefinthermoplastic film substrate which comprises an aqueous natural rubberlatex, an addition polymer emulsion, said polymer being capable ofkeying to the substrate, being non-tacky and incompatible with naturalrubber, and the weight of solid addition polymer being in the range 20to 80% of the total solid material present.

In pressure-sensitive adhesives according to the present invention, thepresence of the addition polymer, apart from controlling the tack of theadhesive, also increases the adhesion of the adhesive to the substrateand tends to prevent the rubber stretching when the bond is broken. Byvarying the ratio of natural rubber latex to addition polymer, it ispossible to obtain a substantially non-tacky coating which seals underhigh pressure, such as that developed when the coated film is passedthrough crimprollers, to a relatively tacky coating which seals underlow pressure. The weight of solid addition polymer present is in therange 20 to 80% of the total solid material present.

The addition polymer is capable of keying to the polyolefinthermoplastic film substrate, is non-tacky. i.e', dried coatings of thepolymer do not form cohesive bonds when pressed together, and isincompatible with the natural rubber, i.e., is incapable of forming ahomogeneous composition therewith.

The ability of an addition polymer to key to a substrate is mostconveniently tested by mixing the addition polymer in aqueous emulsionform with a modified natural rubber latex as described hereafter inExample 2. The quantity of addition polymer emulsion added is such thatthe weight of polymer present is approximately equal to the weight ofnatural rubber present. The mixture thus produced is applied to a stripof the substrate one inch in width and allowed to dry-to form anadhesive coating. Two such coatings are then sealed together by passingthe strips through a crimp wheel device giving a pressure of between 300and 400 lbs. per square inch. The strength of the bond is thendetermined by clamping one strip and attaching a scale pan to the other.25 gram weights are then added to this pan at 10 second intervals. Thetotal Weight added when the bond parts is taken as the bond strength. Ifthis is greater than grams the addition polymer must have been capableof keying to the substrate.

Examples of suitable addition polymers are polymers, copolymers andterpolymers of esters of unsaturated acids containing 3 or 4 carbonatoms, e.g., an acrylate ester or a methacrylate ester, said esterspreferably being derived from lower aliphatic alcohols (containing up tosix carbon atoms). Specific examples are ethyl acrylate/ methylmethacrylate/styrene terpolymers; acrylate ester/ methacrylate estercopolymers; acrylate ester/styrene copolymers; polymethyl methacrylateand polybutyl acrylate. Preferred terpolymers can have a composition inthe following weight range: styrene 5244%, methyl methacrylate 274l%,and ethyl acrylate 2115%.

According to the present invention, preferred pressuresensitiveadhesives may be made up from a modified natural rubber latex having thefollowing composition, the percentages quoted being by weight:

Natural rubber latex 72.9-95.15 Thickening agent 4.525%. Antioxidant0.1l%.

Wetting agent 0.050.l%.

Foam suppressor 0.21.0%.

Alkali Sufilcient to produce a pH value of 9.5l0.0.

Various thickening agents may be used such as alginates, solublecellulose derivatives, e.g., carboxy methylcellulose and methylcellulose, casein, ethylene oxide polymers and water soluble acrylatepolymers. The thickening agent or agents is used together with addedwater (if necessary) to produce a liquid capable of workingsatisfactorily on various types of applicator machines, e.g., spraycoaters, stencil machines and curtain coaters.

The adhesives may contain tackifying resins, e.g., terpene resins, butsuch are not essential components of the adhesives according to thepresent invention.

The tack of the adhesives according to the present invention can bereduced by the addition of antiblocking agents. Suitable proportions areup to 10% by weight of the solid content of the adhesive. Examples ofsuch agents are polyethylene and parafiin and amide waxes.

The pressure used to press the surfaces to be sealed together withadhesives according to the present invention varies with the nature ofthe pressure-sensitive adhesive. The adhesive can be formulated so thatvery light pressures do not cause adhesion and thus accidental touchingof coated surfaces will not cause seals to be formed. Adhesives can beformulated which will seal films under relatively light pressures, i.e.,finger pressure, and which can be broken and remade several times. Theseadhesives are particularly useful in packaging applications where thepacket has to be opened and rescaled using finger pressure only.

The adhesives according to the present invention are particularly usefulfor use with a polyolefin thermoplastic film substrate, e.g., orientedfilm made from polypropylene having a high isotactic polypropylenecontent. Such films may readily be bonded together by means of theadhesive according to the present invention or to other substrates suchas paper, aluminium foil or polystyrene film.

The bond strengths of adhesives according to the present invention areincreased on polyolefin thermoplastic films by subjecting the surface ofthe film to corona discharge treatment. In the case of orientedpolypropylene film the highest bond strengths are formed when the coronadischarge treatment has been such that the treated surface is wetted bysolutions having a surface tension of between 38 and 41 dynes percentimetre.

The following examples describe pressure sensitive adhesives accordingto the present invention.

EXAMPLE 1 A pressure adhesive formulation is prepared by mixing:

Parts by weight 60% natural rubber latex 65 60% sodium dioctylsulphosuccinate 0.08 Casein (thickening agent) 4.5

A styrenated phenol sold under the trade name "Ienaxatex 3001 A is astyrene/methyl methcrylate/ ethyl acrylate terpolymer emulsioncontaining of a polyethylene wax. Solids content is 59%. It ismanufactured by H. A. Smith Ltd., Great Britain.

This adhesive is coated onto 50 gua-ge biaxially oriented polypropylenefilm using an air knife coating technique to give a coating of 0.0003inch thickness. The adhesive is dried at 130 C. in hot air to give asubstantially clear coating. This coating can be sealed to itself usinghigh pressure, preferably that provided by a crimp wheel device. Whenthe bond is broken the rubber in the coating breaks off short, i.e., itdoes not stretch; this is due to the terpolymer. The coating wassubstantially non-blocking as shown by the fact that a piece of coatedfilm did not stick to an uncoated sample, when placed under a 500 gm.weight for 24 hrs. at C. By reducing the amount of terpolymer present inthe mixture within the range 20 to 80% of the total solid materialpresent a tackier coating can be obtained which seals under lowpressure, i.e., just by touching two coated surfaces together.

The terpolymer is capable of keying to biaxially oriented polypropylenefilm and is non-tacky. It is also incompatible with natural rubber ascan be demonstrated by the following procedure:

Two microscopic slides were coated with films (0.003 inch in thickness)of the adhesive described above and allowed to dry in air at 60 C. Thetwo films were then coated with glycerine and the slides placed togetherwith the coated surfaces together. The degree of haze caused by the twofilms was then measured by a standard optical technique. The aboveprocedure was then repeated but in this case one slide was coated with acomposition corresponding to the adhesive from which the additionpolymer emulsion had been removed and the other with the additionpolymer emulsion on its own. The degree of haze was again measured. Itwas found that the degree of haze was greater in the films made from theadhesive composition, thus demonstrating that the terpolymer wasincompatible witi. the natural rubber.

This testing procedure is a convenient one to employ when the additionpolymer has a refractive index difierent from that of natural rubber anda particle size suflicient to be detected by the optical techniqueemployed.

4 EXAMPLE 2 A modified natural rubber latex was prepared having thecomposition:

Parts by weight 60% natural rubber latex 65.0 Ammonium hydroxide (.880)0.8

A styrenated phenol sold under the trade name To 60 parts by weight ofthis was added 40 parts by Weight of a terpolymer of ethylacrylate/methyl methacrylate/styrene containing 10% of a polyethylenewax, which is sold under the trade name Tenaxatex 3001/A.

The adhesive thus produced was applied to a film of isotacticpolypropylene which had been treated by a corona discharge to give a wetadhesive coating of 0.0015 inch thickness. The film was dried at C. in astream of air and reeled onto a 1'' diameter cardboard core to give asubstantially clear coat. The coating was sealed to itself under apressure in the range 300400 lbs/sq. in. by passing through a crimpwheel device. The bond gave a peel strength of 275 grams per inch. Thecoating was substantially non-blocking.

EXAMPLE 3 Parts by weight The modified natural rubber latex described inExample 2 45 Texicryl AS 530 55 Parncol 404C 2 EXAMPLE 4 Parts by weightThe modified natural rubber latex described in Example 2 75 Vinamul N30037.5

Vinamul N300 is a polymethyl methacrylate emulsion manufactured by VinylProducts Ltd., Great Britain, containing 50% by weight of polymer. Thepolymethyl methacrylate is incompatible with rubber, non-tacky andcapable of keying to isotactic polypropylene film. Tested as describedin Example 2 the adhesive gave a peel bond strength of 300 grams perinch. The coating was completely non-blocking but opaque.

EXAMPLE 5 Parts by weight The modified natural rubber latex described inExample 2 25 Tenaxatex AM-278 75' Dribrite 5 Tenaxatex AM278 is anacrylate/methacrylate copolymer emulsion manufactured by H. A. SmithLtd., containing 50% by weight of copolymer. The copolymer isincompatible with rubber, non-tacky and capable of keying to isotacticpolypropylene.

Tested as described in Example 2 the adhesive gave a peel bond strengthof 350 grams per inch. The coating was substantially non-blocking andclear.

By way of comparison with Examples 2 to 5, the modified natural rubberlatex described in Example 2 gave the following results. When coatedonto the polypropylene film the dried rubber has very poor keying to thesubstrate, also reels wound from this coated film block badly. The badkeying can be demonstrated by forming a rubber to rubber bond. When thisis parted the rubber is removed from the substrate.

We claim:

1. A pressure-sensitive adhesive for a polyolefin thermoplastic filmsubstrate which comprises an aqueous natural rubber latex and aterpolymer having a composition in the following weight range: styrene52-44%, methyl methacrylate 27-41%, ethyl acrylate 21-15%, the weight ofterpolymer being in the range 20-80% of the total solid materialpresent.

2. A pressure-sensitive adhesive according to claim 1 wherein apart fromthe terpolymer the adhesive has the following composition, thepercentages quoted being by weight:

Natural rubber latex 72.9-95.15%. Thickening agent 45-25%. Antioxidant0.1 to 1%.

Wetting agent 0.5 to 0.1%.

Foam suppressor 0.2 to 1.0%.

Alkali Sufficient to produce pH value of 9.5 to 10.0.

References Cited UNITED STATES PATENTS 2,963,452 12/1960 Sinn et al 26043,230,186 1/1966 Kreibich 2604 2,929,795 3/1960 Reid et al 26043,092,250 8/1960 Knutson 260888 3,154,430 10/1964 Goodloe et al. 2604FOREIGN PATENTS 874,240 8/ 1961 Great Britain.

MURRAY TILLMAN, Primary Examiner.

M. I. TULLY, Assistant Examiner.

